Aircraft landing gear are subjected to various loads and forces during operation of an aircraft. In addition to the obvious loads associated with landing any aircraft, landing gear are subjected to other forces during ground movement or taxi as the aircraft moves between landing and takeoff. Nose landing gear, which are the most common attachment location for the tugs or tow vehicles used to push an aircraft back from a gate upon departure or to move an aircraft without engine use, are also used to steer an aircraft during taxi and are, therefore, subjected to forces and loads associated with the movement of an aircraft by an external vehicle. In a commercial-sized aircraft weighing in excess of 100,000 kilograms (kg) or 50,000 pounds (lb), these loads and forces can be considerable. Consequently, aircraft landing gear structures, particularly nose landing gear structures used to steer aircraft and attach tow vehicles, must be designed to withstand the range of loads and forces likely to be encountered when an aircraft is moved on a ground surface.
Aircraft landing gear are specifically designed to endure a range of forces encountered during all phases of aircraft operation affecting the landing gear. The arrangement of structural components and the materials from which the components are made are selected to avoid adding unnecessary weight while enhancing operating safety in view of loads likely to be encountered during landing, taxi, and takeoff. The forces on landing gear structures will be different at different times. For example, when an aircraft turns during taxi, upward forces act on landing gear structures, and when the aircraft is traveling in a straight line, lateral forces act on landing gear structures. Aircraft landing gears are designed with specific structures, for example torque links and the like, to absorb such forces and prevent damage to the landing gear.
Once an aircraft has touched down, loads and forces an aircraft will encounter are associated with taxi movements of the aircraft on the ground between its touchdown location and its arrival location and then between its departure location and takeoff. At the present time, moving an aircraft on the ground requires operation of at least one of the aircraft's main engines to enable the aircraft to taxi from the touchdown location to an arrival gate or other parking location. Upon departure of an aircraft from the gate or parking location, the aircraft is attached to a tug or tow vehicle, generally with a tow bar, but at some airports with a towbarless tug, and pushed or otherwise moved in reverse away from a gate or other parking location to a location where the aircraft can safely activate at least one of its main engines and move in a forward direction to the takeoff location. Other aircraft ground movement, such as, for example, the movement of an aircraft to a hangar for maintenance or repair, is typically accomplished by attaching the aircraft through a tow bar to a tug or tow vehicle that pulls the aircraft on the ground, but could also be done by operating the aircraft's engines.
As indicated, tugs and tow vehicles are most often attached to an aircraft's nose landing gear. Tug attachment occurs at specifically designed tow fittings formed integrally with or securely attached to a suitable landing gear structural element. Less frequently, a tug or tow vehicle is attached to an aircraft's main landing gear. Whether an aircraft's nose landing gear or main landing gear is the site of tug attachment, tow fittings must be designed to withstand and absorb towing loads on the aircraft's landing gear, since the tow load is transmitted through the tow fittings.
Because an aircraft's nose landing gear, rather than the main landing gear, is the principal site of tug attachment and is most often subjected to towing loads, nose landing gear structures must be designed and tested to withstand these towing loads. Tow fittings are typically placed in locations on nose landing gear where tug attachment and towing can be accomplished most effectively. Most tugs require attachment of a tow bar to these tow fittings to tow an aircraft. Towing loads are measured, tested, and analysed through the tow fittings normally found on an aircraft's nose landing gear. Certification of tow fittings and other load-absorbing or force-transferring landing gear structures by the Federal Aviation Administration (FAA) and corresponding international regulatory authorities indicates that the design of these structures conforms to required standards and can withstand loads produced during landing gear operation. Relocation of the tow fittings, driving an aircraft differently on the ground, or towing an aircraft in a different way, such as, for example, by a towbarless tug that lifts an aircraft's nose landing gear off the ground for towing, requires reassessment of the forces and loads applied to landing gear and landing gear structure under these changed conditions. Such forces and loads could differ significantly from those to which landing gear structures were subjected prior to modification. Landing gear design and analysis to address the changed torque loads also becomes more challenging. Simply retrofitting aircraft landing gear with different torque or force connection points without a potentially lengthy certification process before the FAA or corresponding international aviation authorities is not a likely possibility.
The use of a drive means, such as a motor structure, integrally mounted with a wheel to rotate the wheel of an aircraft landing gear and move the aircraft has been proposed. The use of such a structure should move an aircraft independently and efficiently on the ground without reliance on the aircraft's main engines. U.S. Pat. No. 7,445,178 to McCoskey et al, for example, describes an aircraft ground movement system with electric nose wheel motors that work in concert with a surface guidance system intended to move a taxiing aircraft. This system requires ground based and other components external to the aircraft for its operation, however. U.S. Pat. No. 7,469,858 to Edelson; U.S. Pat. No. 7,891,609 to Cox; U.S. Pat. No. 7,975,960 to Cox; U.S. Pat. No. 8,109,463 to Cox et al; and British Patent No. 2457144, owned in common with the present invention, describe aircraft drive systems that use electric drive motors to power aircraft landing gear wheels and move an aircraft on the ground without reliance on aircraft main engines or external tow vehicles. While the drive means described in these patents can effectively move an aircraft without towing or aircraft main engine operation during ground operations, transmitting drive forces produced by the drive means through existing landing gear components is not suggested. None of the foregoing art, moreover, recognizes the significant improvement and simplification in landing gear design and load analysis possible with the present invention.
A need exists for a system and method for efficiently equipping an aircraft landing gear with non-engine drive means controllable to power landing gear wheels to move an aircraft autonomously on the ground without attachment to tow vehicles or reliance on aircraft engines that uses existing landing gear structures to distribute loads and forces produced during aircraft ground movement.